What can happen?
1. If the contact points aren't aligned (since the nano SIM Card would be loose), the SIM won't work;
2. Depending on which model of the HTC Desire you're referring to, you could either:
2.a damage the SIM port or
2.b get the SIM stuck inside the port.
3. You could damage the SIM itself.
4. It could work, but only temporarily - depending on how well the SIM is placed, a slight knock could misalign the SIM from the contact points.

What can happen?
1. If the contact points aren't aligned (since the nano SIM Card would be loose), the SIM won't work;
2. Depending on which model of the HTC Desire you're referring to, you could either:
2.a damage the SIM port or
2.b get the SIM stuck inside the port.
3. You could damage the SIM itself.
4. It could work, but only temporarily - depending on how well the SIM is placed, a slight knock could misalign the SIM from the contact points.